You could still use broadcast frame data, but power line cycles are a pretty good choice and can be very easy to sense/amplify as long as you can deal with the lower frequency. You can also use cellular time, since every tower has pretty precise timekeeping if you have a radio to interface with it. You've also got old fashioned radio time signals, though at least in the US, some of the older standards have been/are being phased out.
I'd assume any AM commercial broadcast would be good too - their frequency is going to be tightly disciplined and spot on, and since there isn't any frequency variation component to the modulation, it should be easy to extract the carrier.
You could extract frame timing info in a similar way from TVs nowadays, but it's going to be the local refresh timing based off the internal oscillator, I suspect.